The present invention relates to electrically actuated shifting mechanisms for automated mechanical transmissions. Specifically, the invention relates to an electronic circuit used to compensate for variations in the calibration of transmission shifters in automobiles or other vehicles as a result of temperature variations in the related electronic circuitry. The invention teaches an electronic circuit and method for providing a feedback calibration reference signal to a microprocessor from a power supply circuit that allows the microprocessor to account for variations in power supply temperature and resulting measurement variations during the shifter calibration process.
Electrically actuated X-Y shifting mechanisms for effecting gear shifts in automated mechanical transmissions are well known in the art. Such mechanisms require positioning calibration in order to insure that operation of the mechanism produces accurate gear shifts. Various methods and algorithms exist for calibrating X-Y shifting mechanisms, and one such method is taught in U.S. Pat. No. 5,350,240, which is assigned to the assignee of the present invention, and incorporated herein by reference. The calibration algorithms are typically performed by a microprocessor that uses input values from one or more position sensors.
However, the outputs of position sensors are sensitive to variations in the temperature of the related electronic circuitry and the input power supply. When the temperature of related electronic circuitry and power supply to the position sensors is elevated as compared to ambient conditions, the outputs of the position sensors will be different from the outputs when such components are at a lower temperature, even though the shifter is in the same physical position. Accordingly, it is well-known in the art for the microprocessor to utilize a calibration reference signal to compensate for these temperature-dependent offsets in position sensor outputs.
In the prior art, the calibration reference signal is taken directly from the bias voltage signal that powers the position sensors. However, most microprocessors are intolerant to voltage variants above 0.5 volts. Accordingly, most systems require voltage surge protection circuitry between the calibration signal and the microprocessor. One of the problems with this methodology is that the additional circuitry may itself offset the calibration reference signal as a result of its own variations in temperature. Thus, it is desirable to have an electronic calibration circuit that includes a calibration reference signal that is treated as one of the control variables measured at the same time as the position sensors and fed directly to the microprocessor without any intervening circuitry.
The present invention is directed to an electronic circuit used to calibrate a gear shifter in a transmission. Specifically, the present invention compensates for variations in the measurements provided by the transmission position sensors resulting from differences in the temperatures of the related electronic components at different times. For example, for a vehicle transmission shifter calibrated when the vehicle and related electronic circuitry are at an elevated temperature as compared to ambient conditions, the measurements provided by the shifter position sensors will be different than when the vehicle and related circuitry are at a lower temperature, even though the transmission shifter is in the same physical position. The present invention compensates for that difference and adjusts the transmission shifter position data based upon a feedback calibration reference signal provided to the microprocessor from the power supply circuit.
The electronic circuit of the present invention includes a microprocessor for implementing the calibration algorithm for physically adjusting the transmission shifter relative to the various inner wall surfaces of the shift block. The present invention can be used in connection with a variety of calibration algorithms. One such algorithm is taught in U.S. Pat. No. 5,305,240. As input to the calibration algorithm, the microprocessor receives shifter position data from one or more shifter position sensors. The position sensors are powered by a power supply circuit, which includes a bias voltage supply and a voltage surge protection circuit. The voltage surge protection circuit is disposed between the bias voltage supply and the position sensors to protect the bias voltage supply and position sensors from short circuits or transient voltage or surge voltage conditions. A calibration reference signal is measured from the voltage surge protection circuit. The calibration reference signal is derived by scaling the output bias voltage supplied from the bias voltage supply to the position sensors. Because most microprocessors are intolerant to inputs that vary more than 0.5 volts, it is important that microprocessor inputs be protected from voltage surge or transient conditions. However, in this invention, because the calibration reference signal is produced by the voltage surge protection circuit, no additional voltage surge protection is necessary to protect the microprocessor from large swings in input voltage from the calibration reference signal. Accordingly, the calibration reference signal is input directly from the voltage surge protection circuit to the microprocessor through an A-D converter. A corresponding calibration reference signal is fed back to the microprocessor each time the position sensors measure and input position data to the microprocessor.
The voltage surge protection circuit of the present invention includes a switch network that initially determines if the position sensors are connected to the circuit and whether a short-circuit or excess current condition exists. If there is no short-circuit or excess current condition, the switch network permits the system to xe2x80x9cpower upxe2x80x9d. If a short-circuit or excess current condition does exist, the switch network prevents the system power supply from providing power to the system. Thus, the bias voltage supply is protected from possible damage from the short-circuit or excess current condition.
The voltage surge protection circuit also includes an output current control circuit for controlling the current provided from the bias voltage supply during normal operation and for cutting off the bias voltage supply if a short-circuit or excess current condition is detected during operation. In a preferred embodiment, the output current control circuit includes a bi-polar junction output transistor connected between the system power supply and the load device. The output current of the output transistor depends upon a drive current control signal, which is the output of a drive current control circuit. Preferably, the drive current control circuit includes a pre-drive transistor, which controls the input current to the base of the output transistor, which in turn dictates the output current supplied to the position sensors.
During normal operation (i.e., when there is no short-circuit or an excess current condition), the pre-drive transistor determines a stable level of output current to deliver to the position sensors by receiving feedback from the output transistor. It is generally preferred that the output voltage across the position sensors be compared to a pre-determined reference voltage by an operational amplifier. The output of the operational amplifier provides the feedback to and activates the pre-drive transistor. As the output voltage across the load device approaches the pre-determined reference voltage, the currents through the pre-drive transistor and the output transistor decrease until the output voltage stabilizes.
If a short-circuit or excess current situation occurs, the drive current control signal deactivates the output transistor, cutting off all current flow to the position sensors. The output transistor remains deactivated until the short-circuit or excess current situation is eliminated, at which time, the switch network reactivates the circuit.
During system operation, the position sensors measure the physical position of the transmission shifter and input the position data to the microprocessor through an A-D converter. When the vehicle is turned off, the last set of shifter position data and the corresponding calibration reference signal are stored in a memory device. When the vehicle is turned back on and the vehicle electronics are at an ambient temperature (xe2x80x9ccoldxe2x80x9d), the shifter position data measured by the sensors will be different from that which was measured when the vehicle was at an elevated operational temperature (xe2x80x9chotxe2x80x9d), even though the physical position of the transmission shifter is the same. The microprocessor compares the calibration reference signal stored when the xe2x80x9chotxe2x80x9d position data was measured with the calibration reference signal measured when the vehicle is cold. Based upon the difference in the calibration reference signals, the microprocessor adjusts the stored shifter position data. Because the calibration reference signal is input directly to the microprocessor from the voltage surge protection circuit, the potential for offset variants of the calibration reference signal resulting from additional surge protection circuitry between the calibration reference signal and the microprocessor is eliminated.